Can you explain the difference between these two sentences?
I cannot go to school.
I can not go to school.
I cannot go to school
I can not go to school
In British English, the accept negative form of 'can' is cannot (one word). I cannot (often contracted to I can't especially in speech) go to school almost certainly means I am unable to got to school / It is not possible for me to go to school. It could also mean I may not (= am not allowed to) go to school, though this would need to be made clear by the context.
The 'ability/possibility' meaning is shown in:
I cannot (can't) go to school) because we are snowed in.
The 'permission' meaning is shown in:
My father says I cannot (can't) go to school. He says that education is only for boys.
I can not go to school is more problematic. It could be that the person who wrote this has simply not written the two words as one. This is a very common 'mistake', because 'not' is not written together with the verb that precedes it with any other verb. Indeed, some people would not consider this to be a mistake. Note that we cannot tell in speech the difference between 'can not' and 'cannot'. Both forms can be contracted to 'can't'.
A second, less common, possibility for 'can not' is that it means 'have the ability not to'. In this sense, the two words are always written separately. In speech,, there is a slight but discernible pause between them, and the 'not' is stressed. An example of this is:
A: You have to go to school tomorrow. You can't not go.
B: I can not go. Nobody can make me.
Note that, as we see from A's words, this form can have a 'double negative; You can't not go (It is not possible/permissible for you not to go) does not mean the same as You can go.